The present invention relates to a transistor amplifying means and, more particularly, to an emitter-grounded high frequency amplifier circuit with a bias circuit.
A conventional amplifying means known as a voltage feedback type self-biasing circuit is diagrammatically shown in FIG. 1. As shown therein, a transistor Q.sub.1 has its collector connected to a signal output terminal 12 and connected to a power source terminal 10 through a load 8, its emitter grounded, and its base connected to a signal input terminal 11 and also connected to its collector through a feedback resistor 30.
FIG. 2 is a circuit diagram showing another amplifying circuit known as a current feedback type self-biasing circuit. In this circuit, a voltage divided by serially-connected resistors 41, 42 is applied to the base of a transistor Q.sub.1 as a bias voltage, and the current is fedback with the resistor 43 being connected to the emitter thereof. A capacitor 44 connected to the emitter is provided such that this emitter is grounded for AC components of an input signal applied at the input terminal 11.
In the conventional circuit shown in FIG. 1, the base-collector voltage is given by the product of the base current and the feedback resistance and, if this is small, the transistor operates in a condition which is close to a saturation state, which results in deterioration in high frequency characteristics. A way to prevent the transistor from becoming the saturation state is to increase a resistance value of the feedback resistor 30. For instance, for the collector-base voltage to be 2 V with the collector current being 5 mA and the base current being 50 .mu.A, it is necessary for the resistance value of the feedback resistor 30 to be as high as 40 k.OMEGA.. However, in the integrated circuit device, it is generally difficult to fabricate a resistor having such a high resistance value.
The conventional amplifier circuit shown in FIG. 2 involves disadvantages in that it requires an AC-bypass capacitor 44 and, since the emitter is grounded through the resistor 43 and the capacitor 44, high frequency characteristics and noise characteristics are inferior as compare with those obtained where the emitter is directly grounded.